This invention relates to a washing machine in which a brushless motor is employed as a washing machine motor.
In conventional washing machines, wash and rinse steps are performed by driving an agitator by a washing machine motor and a dehydration step is performed by driving an inner rotatable tub at a high speed by the motor. An induction motor has conventionally been employed as such a washing machine motor.
Small-sized and light-weighted washing machines with high-level cleaning performance have recently been desired. These needs require a washing machine motor of high output torque. However, the induction motor conventionally employed as the washing machine motor is of a relatively large size and the output torque thereof is at a relatively low level. Accordingly, in order to obtain a high output torque from the induction motor, a rating power of the induction motor needs to be increased and therefore, the size of the induction motor needs to be increased, which further increases the size of the washing machine. In view of the disadvantage described above, it has been proposed by the inventors that a brushless motor which is small in size but produces a high output torque be employed as the washing machine motor, instead of the induction motor.
However, when the brushless motor is employed as the washing machine motor, the following deficiency could be found. That is, the agitator or the inner rotatable tub of the washing machine is rotated at a predetermined number of revolution. Since the change in the number of revolution is sufficiently small with respect to load variations in the case of the induction motor, it is easy to control the number of revolution of the induction motor so that it is maintained at a predetermined value. On the other hand, in the case of the brushless motor, the number of revolution changes to a large extent with variations in an amount of load even though a voltage applied to the brushless motor does not vary. To solve this problem, it is usually considered that feedback control of the number of revolution of the brushless motor should be provided. In this feedback control, the number of revolution of the brushless motor is sensed and then, the voltage applied to the brushless motor is varied based on the deviation between the sensed number of revolution and a target number of revolution. Varying the applied voltage is continued until the sensed revolution number reaches the target revolution number.
In the above-described feedback control, however, the present number of revolution is controlled so as to be increased and decreased to the target number of revolution. Accordingly, the actual number of revolution of the motor unstably oscillates relative to the target number of revolution repeatedly, that is to say, the actual number of revolution of the motor is alternately increased and decreased repeatedly so as to sequentially converge to the target number of revolution. Further, the amount of load applied to the motor varies because of rotative movement of clothes caused as the result of drive of the agitator in the wash or rinse step or movement of the clothes due to rotation of the inner tub in the dehydration step. In this case, too, the actual number of revolution of the motor unstably oscillates repeatedly. Consequently, a disadvantage arises that an expected cleaning effect or dehydration effect cannot be achieved.